This invention generally relates to voltage regulation, and more particularly, to a linear voltage regulating circuit with undershoot minimization and a method thereof.
A regulator, coupled between a voltage supply source and a load device, is used to provide a sufficiently constant output current to maintain the drive of a load device. When the load device undergoes a rapid load current transition, where current draw or load impedance alternates between a heavy load and light load, a typical regulator can have several shortcomings. FIG. 1 illustrates such a typical voltage regulator 100 according to the related art. This related art voltage regulator 100 suffers from an undershoot problem when the load device undergoes a rapid transition between a heavy load and light load. The voltage regulator 100 includes a pass transistor MPX coupled between a supply voltage VCC and an output voltage VOUT; an amplifier A1 coupled to the pass transistor MPX for controlling the response of the pass transistor MPX by comparing a reference voltage VREF and a feedback voltage VFB; a feedback circuitry connected between the output node VOUT and the amplifier A1 for delivering the feedback voltage VFB. Additionally, the output voltage VOUT, inducing a load current ILOAD, is coupled to a load device modeled by a load resistor RESR and a load capacitor CL.
Due to loop bandwidth limitations in the load transient response of a transition from a heavy load to light load, the voltage regulator 100 is unable to turn off the pass transistor MPX in time. A large current from the MPX therefore results, and acts to immediately charge the load capacitor CL to increase the output voltage VOUT. This forces the voltage regulator 100 to enter a voltage overload condition. Upon stabilization of the voltage overload condition through the regulator loop, the output voltage VOUT should still be high enough to turn off the pass transistor MPX. However, the charge from the voltage overload stored in capacitor CL will undergo an exponential decay through the feedback network established by resistors R1 and R2. During the time interval between the removal of the output current load, and the appropriate response of the amplifier A1, the output voltage remains unregulated. Meanwhile, if the load device consumes the output current, such as in a case of load current ILOAD transitioning between a light load and heavy load, the output current will only be supplied from the load capacitor CL. This consequently decreases the output voltage VOUT.
When the output voltage VOUT is lower than the desired voltage level, the regulator loop can be activated to restore the output voltage VOUT to the desired level. However, due to loop bandwidth limitations, the output voltage VOUT will supply an undershot voltage to the load device before the pass transistor MPX can be turned on. Moreover, in turning on the pass transistor MPX that is initially turned off, the large gate capacitance of the pass transistor MPX will consume a large amount current. This acts to further worsen the undershot output voltage VOUT. An undershoot output voltage VOUT can therefore seriously hinder the operation of a load device.
U.S. Pat. No. 5,894,227 teaches a voltage regulator utilizing a comparator C1 to compare the gate voltage of the pass transistor and a reference voltage VTRIP in order to control a discharge transistor MPD. However, due to variations in fabrication processes, the reference voltage VTRIP may be set too high. This affects the operation of the discharge transistor MPD, and degrades the overall voltage regulation efficiency under a light load.
Other related art voltage regulators, such as that described in U.S. Pat. No. 5,966,004 and U.S. Pat. No. 6,201,375, utilize a regulator loop with an offset voltage to turn on the discharge transistor when the output voltage is higher than a reference voltage. Although the regulator loop may quickly discharge an initial output voltage, this voltage regulator still suffers from the same problem as described above. When the output voltage becomes lower than the reference voltage, the discharge path is identical to that mentioned in U.S. Pat. No 5,894,227. Since the discharge path still comprises a resistor network, recovery from an unregulated voltage condition may not be any faster due to the regulator loop.